Apparatus, system and method for power measurement at a crank axle and crank arm

ABSTRACT

A power measurement assembly mounted within an axle. In a specific example, the axle is a spindle that is interconnects the cranks of a bicycle, exercise, bicycle, or other fitness equipment. The power measurement assembly may include strain gauges connected with an appropriate circuit (e.g., Wheatstone bridge) that provides an output of the force on the axle by a rider pedaling the crank. In the case of an axle, the strain gauges measure the torsion due to the applied torque on the crank. The value is converted to a power value by a processor and that value is then wirelessly transmitted for display. The processor and/or the transmitter may be mounted within the axle. A separate power measurement assembly may be mounted on one of the cranks, which may include its own processor and transmitter or may take advantage of the processor and transmitter within the axle.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of co-pending U.S. patentapplication Ser. No. 15/925,520 titled “Apparatus, System and Method forPower Measurement at a Crank Axle and Crank Arm” filed Mar. 19, 2018,which is a continuation of U.S. patent application Ser. No. 14/011,297titled “Apparatus, System and Method for Power Measurement at a CrankAxle and Crank Arm” filed Aug. 27, 2013, which claims priority under 35U.S.C. § 119 to provisional application No. 61/693,967 titled“Apparatus, System and Method for Power Measurement at a Crank Axle andCrank Arm,” filed Aug. 28, 2012, which are hereby incorporated byreference herein. The present application is also related to and is acontinuation-in-part of co-pending application Ser. No. 15/232,631titled “Apparatus, System and Method for Power Measurement,” filed onAug. 9, 2016, which is a continuation of Ser. No. 13/356,487 titled“Apparatus, System and Method for Power Measurement,” filed on Jan. 23,2012, now U.S. Pat. No. 9,417,144, which claims priority under 35 U.S.C.§ 119 to provisional application No. 61/435,207 “Apparatus, System andMethod for Power Measurement,” filed on Jan. 21, 2011, which are herebyincorporated by reference herein.

TECHNICAL FIELD

Aspects of the present disclosure involve a power measurement device andmethods for calculating power for use with a crank assembly of abicycle, exercise bicycle or other exercise and fitness equipment.

BACKGROUND

Fitness training using a power meter, particularly for bicyclists, isincreasing popular. Power meters measure and display the rider's poweroutput, typically displayed in Watts, used for pedaling. Power meters ofmany different sorts have been adapted for use on bicycles, exercisebicycles and other fitness equipment. Many of these designs are overlycomplicated, prone to error, and/or prone to failure, and also tend tobe relatively expensive. As such, many health clubs have yet to addpower meters to their indoor cycling and exercise bikes, and many ridersfind the expense of adding power to their road or mountain bikeprohibitive.

Often such clubs and riders use heart rate monitors for training and toprovide feedback for a rider, rather than using power meters. Thesedevices also may provide information concerning speed, distancetraveled, and calories, but that information cannot include or be basedupon power measurements and thus may not be as accurate as valuesderived from power measuring heart rate. While providing usefulinformation for measuring performance, is not as good as measuring powerin providing consistent and useful information to the rider. Forexample, when rapidly accelerating or sprinting, heart rate lags behindthe rider's effort whereas power provides a nearly instantaneousreflection of the rider's effort. When a rider is dehydrated,malnourished, tired, sick, injured, or otherwise not in optimal ridingcondition, the rider may conduct a workout at a typical heart rate buttheir power at that heart rate may be considerably less than typical.Thus, the rider can identify and possibly rectify the cause of thenon-optimal condition. Finally, measuring and comparing power over anextended period of training, can help a rider identify training thathelps increase power and those that do not and thereby continuallyrefine and improve their training regimen.

With these thoughts in mind, among others, aspects of the power meterand related power measurement techniques of the present disclosure wereconceived.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than limiting.

FIG. 1 is an isometric view of a crank assembly including a pair ofcrank arms interconnected by a crank axle with a first strain gauge andcircuitry mounted within the axle and a second strain gauge mounted on acrank arm such that torque from both crank arms may be measured andconverted into a power crank arm specific power measurement;

FIG. 2 is a bottom view of the crank assembly and power measurementapparatus shown in FIG. 1;

FIG. 3 is a rear view of the crank assembly and power measurementapparatus shown in FIG. 1;

FIG. 4 is a front view of the crank assembly and power measurementapparatus shown in FIG. 1;

FIG. 5 is a left side view of the crank assembly and power measurementapparatus shown in FIG. 1;

FIG. 6 is a right side view of the crank assembly and power measurementapparatus shown in FIG. 1;

FIG. 7 is a top view of the crank assembly and power measurementapparatus shown in FIG. 1;

FIG. 8 is a front view of a strain gauge assembly in the form of fourdistinct strain gauges (conductor) in two pairs offset about 90 degrees,and with the strain gauges mounted on a flexible foil that is adhered toan inner wall of the axle such that the torsion force (arrows) isaligned with the conductors of one of the pair of gauges and arrangedabout 90 degrees to the opposing pair of the gauge conductors;

FIG. 9 is an isometric view of a crank arm with a power measurementapparatus connected thereto;

FIG. 10 is a front view of the crank arm and power measurement apparatusshown in FIG. 9;

FIG. 11 is a left side view of the crank arm shown in FIG. 9, andparticularly showing the outside of the crank arm from which a pedalwould extend;

FIG. 12A is a top view of the crank arm and power measurement apparatusshown in FIG. 9;

FIG. 12B is a top view of the crank arm and power measurement apparatusshown in FIG. 9, with some components of the power measurement apparatushidden to illustrate internal components;

FIG. 13A is a right side view of the crank arm and power measurementapparatus shown in FIG. 9, and particularly illustrated the inside ofthe crank arm to which the power measurement apparatus is connected;

FIG. 13B is a right side view of the crank arm and power measurementapparatus shown in FIG. 9, and particularly illustrated the inside ofthe crank arm to which the power measurement apparatus is connected,with some components of the power measurement apparatus hidden toillustrate internal components;

FIG. 14 is an isometric view of the inside of the crank arm particularlyillustrating a recess, which may be machined or directly molded into thecrank arm, within which are four resistive reed switches that provide afluctuation in resistance proportional to the force applied on a pedalconnected to the crank;

FIG. 15 s a right side view of the crank arm as shown in FIG. 14 andproviding further detail as to the arrangement of the strain gaugeswithin the recess of the crank arm;

FIG. 16 is a top view of the crank arm and power measurement device withvarious components hidden to illustrate internal components;

FIG. 17 is an isometric view of the crank arm and power measurementapparatus with the cover of the cantilevered portion of the housingremoved;

FIG. 18 is an isometric view of the power measurement apparatus withvarious components hidden;

FIG. 19 is a circuit diagram of a Wheatstone bridge circuit and relatedprocessing components that may be used to provide a voltage outputproportional to the force applied to the crank axle or the crank arm;

FIG. 20A is a diagram depicting various points in the rotation of acrank arm, the various points corresponding to the output voltagewaveform shown in FIG. 20B

FIG. 20B is an output voltage waveform of the Wheatstone bridge circuitof FIG. 19 for approximately one revolution a crank arm;

FIG. 21 is system diagram illustrating the electrical componentspositioned within the crank axle and at the crank arm and within thepower measurement housing in wireless communication with a displaycomputer provided separately from the power measurement apparatus; and

FIG. 22 is a flowchart illustrating one method of calculating andtransmitting power measurements from the power measurement apparatus.

DETAILED DESCRIPTION

Aspects of the present disclosure involve a power measurement assemblymounted within a hollow axle, or spindle, interconnecting a pair ofcrank arms. The crank assembly may be part of an exercise bicycle,indoor cycling bicycle, bicycle, or other form of mobile device orexercise equipment using a crank assembly. A strain gauge is mounted onan inner wall of the axle and configured to measure the torque appliedto the axle. The torque is representative of the torque applied to thecrank arms. Overall, a rider's total power output may be approximated bydoubling the power derived from the torque measurement taken from theaxle. Aspects of the present disclosure may, however, further involve asecond power measurement device or assembly mounted on a crank armadjacent a drive sprocket. In this example, the rider may obtain powermeasurements for each leg (derived from the torque applied to eachcrank). The device also produces a total power output by adding the twopower values derived from the torque on the axle (representative of thecrank arm opposite the sprocket) and the torque on the opposing crankarm adjacent the sprocket.

In one particular implementation, each of the components that measurepower, calculate power, and transmit the power calculation to a display,are mounted within the axle. Alternatively, those components may bemounted on the crank arm. In one particular implementation, the displaywirelessly receives power data and displays power values. The displaymay be mounted anywhere desirable, such as on a handlebar. The displaymay also be incorporated in a wrist watch or cycling computer. Finally,the power data may be transmitted to other devices, such as a smartphone, tablet, lap top or other computing device for real-time displayand/or storage.

More specifically and referring to FIGS. 1-7, a crank assembly 10conforming with aspects of the present disclosure includes a pair ofopposing crank arms (12A, 12B) interconnected by a hollow orsubstantially hollow axle 14, also referred to as a “spindle.” The axleis interconnected with each crank at corresponding apertures (16A, 10B)defined in the respective crank arms. A pedal aperture (18A, 18B) thatreceives various possible styles of pedal is defined at an opposite endof the crank. With respect to the left crank 12A, the inside of theaperture and the outer surface of the end of the axle may definematching ridges such that the crank arm will not rotate relative to theaxle when force is applied. The opposite, right, crank arm 12B ismounted to the axle similarly. However, the right crank arm may define acircumferential flange around the axle to which a drive sprocket 20 ismounted. The drive sprocket carries a chain or belt that drives a rearwheel (in a conventional bicycle), a flywheel (in an indoor cyclingbike) or otherwise. While one sprocket is shown, additional sprocketsmay also be included. Additionally, the sprocket or sprockets (alsoreferred to as chain rings) may be coupled with the axle or crank indifferent ways.

As introduced above, the axle 14 may be hollow and thus defines a tubewith an inner wall. A strain gauge assembly 22 is mounted on the innerwall. FIG. 8 is one example of a printed circuit board 24 having twosets of two strain gauges. In this example, the first set includesstrain gauges 70A and 70B and the second set includes strain gauges 70Cand 70D. As shown, the first set of strain gauges are set at angle andthe second set of gauges are rotated about 90 degrees relative to thefirst set. When the PCB is mounted on inner wall of the axle, as shownin FIG. 8, the torsional forces translated through the axle align witheither the first set of gauges or the second set of gauges. While astrain gauge assembly with two pair of strain gauges (gauge conductors)is illustrated herein, it is possible to use two gauges or even only onegauge. The arrangement shown, however, provides more efficienttemperature compensation and other benefits relative to other options.Should more or less gauges be used, then other circuit topologiesbesides a full Wheatstone bridge may be used such as a quarter bridge, ahalf bridge, and for particularly low resistance changes, a Kelvinbridge, as well as others.

As shown in FIGS. 8 and 9, the strain gauges 70 each include leads 76connected in a Wheatstone bridge circuit arrangement. For example, asshown in FIG. 9, the strain gauges are connected in the circuitarrangement shown. Other circuit arrangements that use more or lessstrain gauges are possible, such as a half bridge configuration. Aninput current is applied to the bridge circuit and the output voltage ofthe circuit is proportional to the torsional force (torque) applied tothe crank axle by the rider pedaling the left crank arm (in the exampleshown), as the strain gauges change resistance as they are placed intension or compression. Referring to FIG. 8, when a torsional force isapplied in the direction of the arrow, strain gauges 70A and 70B areplaced in tension and the resultant resistance change is converted to anoutput voltage related to the torque. The output voltage may be appliedto some form of conditioning and amplification circuitry, such as adifferential amplifier and filter that will provide an output voltage tothe processor. It is further possible to use an analog to digitalconverter to convert and condition the signal. As mentioned above, theprinted circuit board supporting the Wheatstone bridge, the processor,conditioning, amplification, wireless transmitter, etc., is supportedwithin the axle. Thus, leads, also within the axle, from the straingauge PCB may extend to the main PCB. Either a torque value that needsto be converted to power, or power values are wirelessly transmittedfrom the transmitter.

With strain gauges mounted within the axle, it is possible to measurethe power associated with one crank. To estimate the total power, thesingle measured value may be doubled. In such an arrangement, relativesymmetry between the right and left leg of a given rider is assumed andthe average power calculated from the power measurement device withinthe spindle is doubled and transmitted to the display processor (ordoubled at the display processor). Alternatively, the crank assembly mayinclude a second power measurement device associated with the opposingcrank. In the example of FIG. 1, the power delivered by the rider to theleft crank 12A is measured within the crank axle 14 whereas the powerdelivered by the rider to the right 12B, opposing, crank is measured bya set of separate strain gauges 26 mounted on the right crank or someother component associated with the right crank. Generally speaking,strain gauges are mounted on the right crank and the leads are connectedto a Wheatstone bridge circuit to generate an output voltage indicativeof torque. The Wheatstone bridge and other processing components may belocated on a PCB 28 within the axle 14. In such an arrangement, leadsfrom the strain gauge mounted on the pedal extend into the axle and areconnected with a separate Wheatstone bridge. Alternatively, and asdiscussed hereafter, separate electronics may be mounted on the crankalong with the strain gauges.

More particularly and referring to FIGS. 9-20 among others, in theexample implementation shown herein, a power measurement device 30 ismounted on a crank arm 32. The crank arm 32 shown is particularly suitedfor an indoor cycling (IC) bicycle; however, the crank arm may be usedon other forms of exercise bicycles, whether upright, recumbent, orotherwise, may be used with bicycles, may be used with other forms offitness equipment that employ a crank arm, such as elliptical trainers,stair climbing machines, and the like, and may be used with any devicethat includes a crank arm and where power measurement or the componentsof power measurement (e.g. torque, force, RPM) may be desired orotherwise beneficial.

The power measurement device 30 includes a housing 34 secured to aninside portion 38 of the crank arm between a bottom bracket aperture 40and a pedal aperture 42. Various power measurement electronics areprovided within the housing. The inside portion 38 of the crank arm,where the housing 34 is mounted, is that portion adjacent or facing thebicycle frame, drive sprocket, etc. In various possible otherimplementations, the housing 34 may also be secured to other portions ofthe crank arm, such as the top, bottom or outside portion. However,securing the housing to the inside portion of the crank arm shields thehousing and attendant power measurement components from inadvertentcontact with a rider or other obstacle. For example, if a rider's footwere to slip off the pedal, the foot could contact the housing if it wassecured to some other portion of the crank arm. However, on the insideof the crank arm, the rider's foot would not contact the housing.

Referring now to FIG. 12-18, the housing 34 includes a mounted portion44 and a cantilever portion 46. The mounted portion 34 is secured, suchas through a pair of bolts 48, to a machined recess 50 in the crank arm.It is also possible to attach the housing 34 to the crank using tape,adhesive or other mechanisms. As discussed further below, one or morestrain gauges are mounted to the crank arm within the machined recess50. The mounted portion 44 defines a male portion with a circumferentialflange 52 such that the male portion is dimensioned to fit snugly withinthe machined recess. A gasket 54 may be provided in a circumferentialchannel defined in the mounted portion adjacent the crank. Whenassembled, the gasket is sandwiched between the mounted portion of thehousing and the crank arm to block moisture, such as sweat from a riderand water or mud from a trail or road, from entering into the recessedarea or into the housing.

The mounted portion further defines a cavity 56 within which areprovided a circuit board 58, reed switch 60 (attached to the circuitboard) and a port 62 by which electrical components on the circuit boardmay be accessed or otherwise communicated with to download software orfirmware updates as well as to access information. Thus, besides thestrain gauge and electrical connections thereto, the various electricalcomponents that process the strain gauge outputs and transmit the dataare located within the cavity of the housing. In one particulararrangement, the pair of bolts 48 extend through the mounted housing andare secured to matching threaded apertures 64 defined in the recessedportion of the crank. The printed circuit 58 board extends between andis connected to a pair of molded cylinders 46 through which the bolts 48pass. The molded cylinders 66 form an integral part of the mountedportion 44 of the housing and extend between an outer wall 68 of themounted portion and the recess in the crank arm. The cylinders may bedimensioned so that it engages the crank and prevents the housing frombeing cracked while tightening the bolts.

The power assembly 30 discussed herein may also be adhered,non-mechanically fastened, to any form of existing crank arm withoutmodifying the crank. In such an example, the power assembly housing mayor may not include a cantilever portion and will not include a maleportion configured to engage a recess. Strain gauges may be adhereddirectly to a particular crank wall, without physical modification ofthe side wall. Some surface preparation (cleaning, etc.) may be requiredbefore adhering the strain gauges to the crank wall, however. A lowersurface of the power assembly housing will define an opening suitable tocover the strain gauges and receive leads connected to the straingauges. Given the vast number of possible crank arms to which theassembly might be adhered, it is possible that the lower surface and/orwall engaging the crank arm surface, may be contoured to match the crankarm wall contour of a given crank arm. Alternatively, a plurality ofdifferent adapters may be fabricated so that a common power assemblyhousing may mate to different crank arms. In such a configuration, andadapter may have a first side that has a matching contour of a givencrank arm, and a second side that has a matching contour of the commonpower assembly housing. The housing in any given configuration includesthe processor, batteries, and wireless transmission capability.Accordingly, the system may be mated to any of a variety of existingcrank arms without modification of the crank arm (e.g., without tappingthe crank arm to accept bolts which could effect the structuralintegrity of the crank), and the power assembly will wireless transmit apower value that may be used to display the power being exerted whileriding and/or exercising on a device including the crank. With such asystem, there is no need to purchase or replace existing drive traincomponents. Rather, a rider may simply retrofit or purchase a crank arm(with power assembly) for his or her existing drive train.

As shown in FIGS. 15, 16 and others, within the recessed portion 50 ofthe crank arm, one or more strain gauges 70 may be provided. In theimplementation shown, two pair of strain gauges are shown with onemember of each pair disposed equidistant from a centerline 72 of thecrank arm to an opposing pair. The strain gauges are placed on theinside wall of the crank arm. In one particular implementation, thestrain gauges are glued to a smooth flat surface of the crank. While amachined or otherwise provided recess is shown, the power measurementapparatus may be applied to an existing crank arm with little or nopreprocessing of the crank arm. The machined recess 50 is provided witha smooth flat bottom upon which the strain gauges are secured. To assistwith consistency between crank assemblies, a template may be used toapply the strain gauges to the crank surface within the machined recess.Alternatively, the strain gauge may be pre-mounted on a substrate in adesired configuration, and the substrate mounted to the crank. The sidewalls of the machined recess also provide a convenient way to locate thehousing.

In the implementation shown, the strain gauges 70 are placed relativelycloser to where the crank is mounted 40 at the bottom bracket 65compared to where torque is applied to the crank arm at the pedal 74. Assuch, with the strain gauges 70 placed relatively closer to the pivotpoint of the crank arm (i.e., the bending point of the theoreticalbeam), there is greater strain gauge output resolution providing alarger output voltage of the Wheatstone bridge circuit, discussedherein, compared to having the strain gauges been placed relativelycloser to the pedal point given the same torque. With greaterresolution, the output voltage is large relative to noise and otherspurious voltage outputs; therefore, the circuit requires relativelyless filtering, amplification and the like to accurately extract thevoltage reading of the circuit.

As shown, the strain gauges 70 may be placed on the same wall of thecrank arm and are arranged in the same relative direction. In oneparticular example, the strain gauges are each parallel to the othergauges. Stated differently, each strain gauge defines a longitudinalaxis across which the strain gauge is response to tension orcompression. Each of the strain gauges is arranged such that thelongitudinal axes are parallel. Hence, in the example of FIG. 15, withthe illustrated downward force on the crank, the upper strain gauges(70A and 70B) will be in tension while the lower strain gauges (70C and70D) will be placed in compression. The arrangement, through itsgeometry, filters out forces not relevant to measuring power applied tothe cranks causing rotation about the bottom bracket. For example,should a transverse force (e.g. normal to plane defined by the 2 pair ofgauges) be applied to the pedal, such as if a rider is applying a forcethat has both downward and non-downward forces on the pedal, then all ofthe strain gauges will compress or tension in the same way from thetransverse force and cause a 0 voltage output of the Wheatstone bridgecircuit. Similarly, non-tangential forces applied to the pedal areautomatically normalized to a tangential force measurement.

Additionally, the strain gauges are positioned on the same wall orsurface of the crank arm 32. In the particular examples set out herein,the strain gauges 70 are each on an inside wall of the crank arm. Theinside wall is the wall facing an opposing crank or otherwise the frameof the exercise bicycle when the crank is assembled on the exercisebicycle. The assembly can be positioned on other walls, depending on theconfiguration. The inside wall, however, provides some protection frominadvertent contact. The inside wall (or opposite outside wall)experiences less deflection during riding as compared to the upper andlower walls (those walls or surfaces connecting the inside and outsidewalls). Placing the strain gauges on those upper and/or lower walls,would provide greater strain gauge bridge output for the same forcesthereby providing potentially higher resolution bridge outputs.Nonetheless, those walls are potentially at risk for much greaterinadvertent contact, whether on an indoor bike or outside bike.

The strain gauges 70 each include leads 76 connected in a Wheatstonebridge circuit arrangement. For example, as shown in FIG. 19, the straingauges are connected in the circuit arrangement shown. Other circuitarrangements are possible that use more or less strain gauges, such as ahalf bridge configuration. With reference to FIG. 21, the strain gauges,processor, and transmitter may be placed in the crank axle or in thecrank housing. An input voltage is applied to the bridge circuit and theoutput voltage of the circuit is proportional to the tangential bendingforce (torque) applied to the crank arm. The output voltage may beapplied to some form of conditioning and amplification circuitry, suchas a differential amplifier and filter that will provide an outputvoltage to the processor. It is further possible to use an analog todigital converter to convert and condition the signal.

With the illustrated strain gauge configuration, the output voltage ofthe Wheatstone bridge circuit is proportional to the torque applied andalso indicative of the direction of rotation and the crank position. Asillustrated in FIGS. 20A and 20B, generally speaking, the output of thesecond Wheatstone bridge will be a sinusoid with the highest outputvoltage with the crank approximately horizontal and a downward forceapplied to the crank arm (crank position A, FIG. 20B). As the crankmoves through the downward vertical position (crank position B), thevoltage will typically be about 0, as the crank moves upward tohorizontal (crank position C) the voltage will be slightly negative.Typically the downward force of the opposing crank arm pushes themeasured arm up against some weight of the rider's leg (typically ridersdo not pull upward on the cranks, the opposing leg hence uses some forceto push the opposing crank arm upward against the opposing leg), and asthe crank moves through the upward vertical position (crank position D)the output voltage will transition from a negative value to a positivevalue, and reach its peak output again as the crank is rotated throughhorizontal (crank position A). The first set of strain gauge and firstWheatstone bridge circuit (associated with the crank axle) will generatea similar sinusoid, except it will be about 180 degrees out of phasewith the sinusoid of FIGS. 20A and 20B because the cranks are 180degrees out of phase.

As discussed above, it is possible that power measurement may occur ononly one of two crank arms are be associated with only one of two crankarms. In such an arrangement, relative symmetry between the right andleft leg of a given rider is assumed and the average power calculatedfrom the power measurement device on one crank is doubled andtransmitted to the display processor. As shown in FIG. 21, a separatedevice may include a wireless receiver, an additional processor and adisplay. In one example, the power doubling occurs within the powermeasurement device (within the crank housing or at the crank), by theappropriate processor or otherwise, and the power value wirelesslytransmitted by the device includes the doubling. With such anarrangement, the power measurement device may work with a proprietarydisplay device or may work with third party devices that implicitlyexpect a value that accounts for both legs and has no inherentfunctionality to double a value. In some examples, the device maywireless transmit the single leg (crank) power value and doubling mayoccur at the display processor or related display electronics.Alternatively, the display may be configured to sum the power valueswireless received, when separate power measurement devices are eachmounted on opposing crank arms to provide distinct crank arm powermeasurements.

Regardless, the power calculations/measurements displayed are indicativeof the total power output by a given rider. Measuring power of only oneleg, while theoretically not as precise as separate devices for eachleg, nonetheless has several advantages. First, retrofitting andmaintaining the power measurement device is far less complicated andcostly compared to a similar implementation with two devices. So, forexample, with respect to the spindle based power measurement, anexisting crank set may simply be retrofitted with a power measurementequipped spindle. Secondly, when installed on training equipment,particularly in a gym environment where a given rider may not use thesame equipment during successive trips to the gym, the device set outherein may be calibrated such that power measurements across machines isconsistent. With such consistency, whether across machines or not, agiven rider can measure overall relative riding differences. Of course,it is possible to apply a power measurement device to the spindle andthe crank arm of a given machine and provide individual crank arm powermeasurements as well as a summation of the power measurements. In suchan implementation, a rider would be provided with data on eachindividual leg as well as a total power output indicative of overallpower output.

A reed switch 60 is included in the power measurement device and amagnet (not shown) may be placed on the frame of whatever device thecrank is attached such that the reed switch closes as it passes themagnet. Thus, the time between two pulses of the reed switch indicateone complete revolution of a crank. Using a microprocessor clock, thepulses can be converted to a revolution per minute measurement.

“Power” is the most common measurement of a rider's strength. Referringto FIG. 22, in one implementation, the instantaneous torque is measuredby the strain gauges with the output voltage of the strain gauges beingconverted to a power value for display. First, the voltage output of thestrain gauges is provided to the processor (operation 1400). Withrespect to the crank arms, the output of the second circuit is a measureof torque and therefore is converted to power through obtaining aradians/sec value. Similarly, the output of the first circuit (relativeto the strain gauges within the axle) is a measure of the torsion on thespindle produced by the crank. Power may be calculated similarly foreither value. With specific reference to an example based upon torquevalue from the crank arms, as mentioned herein, the voltage output mayfirst be converted to a digital value through an analog to digitalconverter. In on specific implementation, power (watts) is calculated asradians/sec multiplied by the torque value, as follows:Power (Watts)=Radians/Sec×Torque.  (1)

The reed switch and/or the accelerometer provide data indicative of eachrevolution of the crank arms. In conjunction with the processor clock,or otherwise, the processor obtains a revolutions per minute value inradians per second (operation 1410). With a reed switch, a pulse isreceive at each revolution of the crank arm, and the pulse to convertedto revolutions per minute through comparison with the processor clock.Then, to convert the RPM value to Radians/Sec, a multiplier value of0.1047 is used, in one particular example, as follows:RPM×0.1047=Rad/Sec.  (2)

The strain gauge measurement, in conjunction with the revolutionmeasurements, is converted to an instantaneous power value by asdiscussed above and multiplying that value by the length of the crankarm between the apertures (operation 1420), as follows:Torque (N*m)=Force (Newton)×Length (meters).  (3)

Hence, with measured torque multiplied by the Rad/Sec value, power iscalculated. In one example, power is sampled at 32 Hz, and converted andwirelessly transmitted (operation 1430) to the display using the ANT+protocol developed by Dynastream Innovations, Inc. While the transmitteris shown as a discrete component within the housing, it is possible thatit, along with other electrical components, might be provided in theprocessor within the axle or the crank housing. Further, the processormay be implemented as an ASIC, as computer executable instructions in amemory attached to the processor, as a customized circuit, etc.Moreover, other protocols and wireless transmission mechanism may beemployed. For example, the transmitter may send Bluetooth messages, andin such an arrangement messages may be sent to the processor with theaxle or the crank since Bluetooth is bidirectional.

In one possible implementation, the average power is displayed(operation 1440) over a number of samples as opposed to an averageacross all samples. In this way, large changes in power associated withrapid acceleration, for example, may be captured but rapidly changingfluctuations between power measurements are filtered by the averaging.In one example, a rolling average of the most recent 64 measurements isdisplayed. Hence, the most recent 64 power measurements are summed anddivided by 64 to display average power. Note, the power measurementdevice transmits instantaneous power measurements at 32 Hz and thosemeasurements are doubled (to account for the opposing crank without apower measurement device). While this combination of transmissionfrequency and averaging is not necessary, it has been shown to beresponsive to instantaneous changes associated with quick accelerationas well as providing a display that is not overly jittery if too fewvalues are averaged. It is also possible to transmit torque values (orvoltages) and RPM measurements, and then calculate power within thedisplay counsel.

Returning again to the figures illustrating the housing 14 (e.g., FIGS.9, 10, and 12) adjacent the mounted portion 44 is a cantilevered portion46 of the housing. The cantilevered portion 46 extends from the mountedportion 44 along the crank arm body toward the pedal aperture 42. Thecantilevered portion 46 houses batteries 78 and connection points toprovide power to the electrical components within the mounted portion ofthe housing. The cantilevered portion defines a substantially flatbottom adjacent, but not touching, the inside portion of the crank arm.Because the crank arm does bend during usage albeit only slightly,cantilevering a portion of the housing provides several advantages.Firstly, by not touching the crank arm, there will not be any squeakscaused by the crank arm moving relative to and rubbing against thecantilevered portion. Secondly, the cantilevered portion is not boltedto the crank arm. Had the cantilevered portion been bolted, the bendingdistance between the bolts on the mounted portion relative to a bolt onthe cantilevered portion could potentially cause the housing to crack,to loosen the bolts, or to damage some internal components.

The cantilevered portion 46 includes a base portion 82 that extends fromand is integral to the mounted portion. The base portion 82 includesopposing side walls 84 and a front wall 86 distal a midwall 88 of themounted portion. Battery contact points 80 are provided at the frontwall and the midwall and batteries are positioned therebetween. Wiresare connected to the contact points and routed to the circuit board toprovide power to the various components coupled thereto. A cover 90 issnap fit to the base portion and may further be secured by a small screw92 engaging a threaded aperture in the front wall. The embodiment shownincludes two AA batteries, which are well suited for a club environmentfor ease of exchange and long life. It is also possible to use smallerwatch style (coin) batteries or other types of power supply.

In an alternative implementation, an accelerometer, which may be a twoor three axis accelerometer, may be used alone or in conjunction withthe reed switch. An accelerometer may be used to provide both crankposition and rpm measurements. Namely, for example, in a two axisaccelerometer or a three axis accelerator where two of the three axesare used, one axis may be aligned with the crank arm and the other axisoriented at 90 degrees to the crank arm. Accordingly, one axis willoutput a value commensurate with the g-force experienced by the crankand the other axis will output a value 90 degrees out of phase with thefirst. By knowing the axis associated with the crank and whetherattached to the left or right crank, the accelerometer, will output avalue that is a function of the crank position among other factors.Further by comparing the output of the other axis, it can be determinedwhether the cranks are being pedaled forward or backward.

In another alternative implementation, a thermistor is operablyassociated with the processor. Strain gauges, the crank material, andother components are affected by temperature. Accordingly, it ispossible that when a power assembly is exposed to significanttemperature variations, such as during a ride that commences early inthe morning and continues as the outside temperature increases, thepower output delivered by the device would vary based on temperature. Insuch situations, the device may output different power values due totemperature even when the rider is cranking at the same power. So, arider cranking along at 150 watts in the early morning would have apower reading of 150 watts, and the same rider cranking along at 150watts after it has warmed up outside may only have a power reading of100 watts. The thermistor can be used to provide temperaturecompensation to the power value and thereby reduce or eliminate thetemperature effect on the power calculation.

In one particular implementation, the Wheatstone bridge circuit providesits voltage output to an analog-to-digital converter to convert thevoltage to a digital value. The thermistor also outputs its voltage tothe analog-to-digital converter to convert the voltage to a digitalvalue. These values are then input to the processor. Therefore, theprocessor receives a digital power value and a digital temperaturevalue.

The processor, which is coupled with a memory and/or includes on-boardmemory, has a power curve and may also have a temperature curve (forthose embodiments including a thermistor). Each curve may be establishedby measuring the output of the A-to-D converter at two known values(e.g., two known forces on the pedal or two known temperatures). Sincethe curves are typically straight line curves, two values are sufficientto determine the slope of the curve. For the power curve, an A-to-Dvalue is compared with the power curve to determine the power beingapplied to the crank. To compensate for temperature, the A-to-D value ofthe thermistor is used to select a temperature offset value (orcompensation value) from the temperature curve to apply to the powervalue.

Although various representative embodiments of this invention have beendescribed above with a certain degree of particularity, those skilled inthe art could make numerous alterations to the disclosed embodimentswithout departing from the spirit or scope of the inventive subjectmatter set forth in the specification. All directional references (e.g.,upper, lower, upward, downward, left, right, leftward, rightward, top,bottom, above, below, vertical, horizontal, clockwise, andcounterclockwise) are only used for identification purposes to aid thereader's understanding of the embodiments of the present invention, anddo not create limitations, particularly as to the position, orientation,or use of the invention unless specifically set forth in the claims.Joinder references (e.g., attached, coupled, connected, and the like)are to be construed broadly and may include intermediate members betweena connection of elements and relative movement between elements. Assuch, joinder references do not necessarily infer that two elements aredirectly connected and in fixed relation to each other.

In some instances, components are described with reference to “ends”having a particular characteristic and/or being connected to anotherpart. However, those skilled in the art will recognize that the presentinvention is not limited to components which terminate immediatelybeyond their points of connection with other parts. Thus, the term “end”should be interpreted broadly, in a manner that includes areas adjacent,rearward, forward of, or otherwise near the terminus of a particularelement, link, component, member or the like. In methodologies directlyor indirectly set forth herein, various steps and operations aredescribed in one possible order of operation, but those skilled in theart will recognize that steps and operations may be rearranged,replaced, or eliminated without necessarily departing from the spiritand scope of the present invention. It is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative only and not limiting. Changes indetail or structure may be made without departing from the spirit of theinvention as defined in the appended claims.

The invention claimed is:
 1. A power measurement assembly comprising: acrank axle; at least one strain gauge mounted directly on an innertubular wall portion of the crank axle; and a circuit connected to theat least one strain gauge and configured to provide an outputproportional to a force applied to the crank axle.
 2. The powermeasurement assembly of claim 1, wherein the at least one strain gaugeis operatively coupled with a printed circuit board and the printedcircuit board is mounted on the inner tubular wall of the tubular wallportion of the crank axle.
 3. A power measurement assembly comprising: acrank axle defining an inner cylindrical wall, the crank axle configuredto be coupled to a first crank arm; a first strain gauge mounteddirectly on the inner cylindrical wall of the crank axle; and a firstcircuit connected to the first strain gauge and configured to provide afirst output proportional to a force applied to the crank axle.
 4. Thepower measurement assembly of claim 3 further comprising a second straingauge mounted on the inner cylindrical inner wall of the crank axle, thefirst strain gauge and the second strain gauge operatively coupled witha printed circuit board, the printed circuit board mounted directly onthe cylindrical inner wall of the crank axle.
 5. The power measurementassembly of claim 3 wherein the first circuit is mounted within thehollow inner portion.
 6. The power measurement assembly of claim 5further comprising a first processor configured to receive the firstoutput from the first circuit, the first processor configured tocalculate a first force value representative of the first force appliedto the first crank arm.
 7. The power measurement assembly of claim 6further comprising a wireless transmitter in operable communication withthe first processor, the wireless transmitter mounted within the hollowinner portion, the wireless transmitter configured to transmitinformation representative of the first force value and the second forcevalue.
 8. The power measurement assembly of claim 4 further comprising athird strain gauge mounted directly on the cylindrical inner wall of thecrank axle and a fourth strain gauge mounted directly on the cylindricalinner wall of the crank axle, the third strain gauge and the fourthstrain gauge operatively coupled with the printed circuit board, whereina first pair of the first strain gauge and the second strain gaugecomprises a first plurality of parallel elongate conductors and a secondpair of the third strain gauge and the fourth strain gauge comprises asecond plurality of parallel elongate conductors oriented about 90degrees relative to the first plurality of parallel elongate conductors,the first plurality of parallel elongate conductors aligned with atorsional force vector induced in the crank axle from the force appliedto the crank axle, and wherein the first circuit comprises a Wheatstonebridge circuit comprising the first, second, third and fourth straingauges.
 9. The power measurement assembly of claim 3 further comprising:a second crank arm defining an outer surface between a pedal aperatureand a bottom bracket aperture; a second strain gauge and a third straingauge mounted on the outer surface between the pedal aperture and thebottom bracket aperture; a housing affixed to the outer surface of thesecond crank arm and disposed entirely at a location between the pedalaperture and the bottom bracket aperture, the housing including a secondcircuit connected to the second strain gauge and the third strain gauge,the housing enclosing the second strain gauge and the third straingauge.
 10. The power measurement assembly of claim 9 wherein the crankaxle defines a hollow inner portion and wherein the first circuit andthe second circuit are mounted within the hollow inner portion, thefirst processor configured to receive the first output from the firstcircuit and the second output from the second circuit, the firstprocessor configured to calculate a first force value representative ofa first force applied to the first crank arm and a second force valuerepresentative of a second force applied to the second crank arm. 11.The power measurement assembly of claim 10 further comprising a displayprocessor and associated display that are configured to receive thepower value from the second processor, the power value being aninstantaneous value that is averaged by the display processor.
 12. Apower measurement device comprising: a crank axle defining a hollowinner portion defined by an inner cylindrical wall to which is directlyattached a printed circuit board comprising at least one first straingauge, the hollow inner portion further including a first circuitconnected to the at least one first strain gauge and configured toprovide a first output proportional to a force applied to the crankaxle, the first circuit coupled with a processor configured to receivethe first output from the first circuit and calculate a first forcevalue, the inner portion further comprising a wireless transmitter inoperable communication with the processor, the wireless transmitterconfigured to transmit information representative of the first forcevalue, the crank axle configured to be coupled to a first crank arm. 13.The power measurement device of claim 12 further comprising a secondcrank arm coupled with the crank axle, the second crank arm defining anouter surface between a pedal aperture and a bottom bracket aperture; atleast one second strain gauge mounted on the outer surface between thepedal aperture and the bottom bracket aperture; a housing affixed to theouter surface of the second crank arm and disposed entirely at alocation between the pedal aperture and the bottom bracket aperture, thehousing including a second circuit connected to the at least one secondstrain gauge, the housing enclosing the at least one second straingauge.
 14. The power measurement device of claim 12 wherein: the atleast one first strain gauge comprises four strain gauges; and the firstcircuit comprises a Wheatstone bridge circuit.
 15. The power measurementdevice of claim 14 wherein the hollow inner portion of the crank axledefines a longitudinal line and a first and second strain gauge of thefour strain gauges are mounted at about a 45 degree angle relative tothe longitudinal line and third and a fourth strain gauge of the fourstrain gauges are offset about 90 degrees from the first and the secondstrain gauges.
 16. The power measurement device of claim 12 wherein thecrank axle is coupled to the first crank arm.
 17. The power measurementdevice of claim 12 wherein the processor calculates a power value. 18.The power measurement device of claim 12 wherein the wirelesstransmitter transmits a force value for translation into a power value.